title: MR elastography of the liver: defining thresholds for detecting viscoelastic changes.
abstract: OBJECTIVE
  To define thresholds for detecting significant change in liver viscoelasticity with magnetic resonance (MR) elastography, both for whole-liver measurements and for voxel-wise measurements in relation to spatial resolution.
  
  
  METHODS
  This prospective study was approved by the institutional review board, and all participants provided written informed consent. Thirty participants (16 volunteers and 14 patients with hepatitis B or C; 18 men; median age, 30.4 years; age range, 18.9-58.6 years) underwent imaging twice while in the same position (intraimage reproducibility), after repositioning (within-day reproducibility), and 1-4 weeks later (between-weeks reproducibility). MR elastography parameters comprised elasticity, viscosity, attenuation parameter α, and propagation parameter β. Bland-Altman analysis was used to calculate repeatability indexes for each parameter. Analyses were performed in a region-of-interest and a voxel-by-voxel level. Voxel-wise results were calculated in relation to spatial resolution by applying Gaussian filtering to establish the optimal trade-off point between resolution and reproducibility.
  
  
  RESULTS
  For elasticity, α, and β, within-day and between-weeks results were significantly lower than intraimage results (P ≤ .018 for all). Within-day and between-weeks results did not differ significantly. Over-time changes of more than 22.2% for elasticity, 26.3% for viscosity, 26.8% for α, and 10.1% for β represented thresholds for significant change. The optimal trade-off between spatial resolution and reproducibility was found at a filter size of 8-mm full width at half maximum (FWHM) for elasticity and propagation parameter β and at 16-mm FWHM for viscosity and attenuation parameter α.
  
  
  CONCLUSIONS
  Repositioning causes a significant decrease in the reproducibility of MR elastography. The propagation parameter β is the most reliable parameter, with an over-time threshold for significant change of 10.1% and the ability to reproduce viscoelasticity up to a resolution of 8-mm FWHM. Online supplemental material is available for this article.
authors.affiliation: From the Departments of Radiology (A.E.B., P.F.C.G., J.S., A.J.N.) and Gastroenterology and Hepatology (A.D.N.), Academic Medical Center, University of Amsterdam, Meibergdreef 9, 1105AZ Amsterdam, the Netherlands; and INSERM Department of Radiology, University Paris Diderot, Sorbonne Paris Cité, INSERM UMR 773, University Hospitals Paris Nord Val de Seine, Beaujon, Clichy, France (P.G., R.S.).
authors.forename: Anneloes E
authors.initials: AE
authors.lastname: Bohte
authors.forename: Philippe
authors.initials: P
authors.lastname: Garteiser
authors.forename: Annikki
authors.initials: A
authors.lastname: De Niet
authors.forename: Paul F C
authors.initials: PF
authors.lastname: Groot
authors.forename: Ralph
authors.initials: R
authors.lastname: Sinkus
authors.forename: Jaap
authors.initials: J
authors.lastname: Stoker
authors.forename: Aart J
authors.initials: AJ
authors.lastname: Nederveen
mesh_terms: D000293:Adolescent; D000328:Adult; D004548:Elasticity; D054459:Elasticity Imaging Techniques / Q000379:methods*; D005260:Female; D006509:Hepatitis B / Q000473:pathology*; D006526:Hepatitis C / Q000473:pathology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D056888:Patient Positioning; D011446:Prospective Studies; D015203:Reproducibility of Results; D013997:Time Factors; D014783:Viscosity